1. Field of the Invention
This invention relates to a composition for and method of making a binder pitch for manufacturing carbon bodies having a substantially homogenous distribution of randomly oriented carbon fibers. The resultant graphite bodies made using the novel binder pitch of the present invention have a desirably lower transverse and longitudinal coefficient of thermal expansion than conventionally made graphite bodies.
2. Description of Related Art
The use of carbon fibers as a filler with pitch as a binder is well known in the art in manufacturing carbon bodies, e.g., graphite electrodes, having a reduced coefficient of thermal expansion (CTE). Typically, carbon bodies having a low CTE are made by admixing an oriented needle-like coke with a thermoplastic carbonizing binder, such as coal tar pitch, extruding or molding the resulting mixture into a desired shape then carbonizing and graphitizing the body. Although the carbon bodies produced in this manner have a low CTE, means for further reducing the CTE are constantly sought to improve the performance of these articles in the high temperature surroundings in which they are employed.
British Patent No. 1,526,809 to Singer et al. discloses an extruded carbon article prepared using 50% to 80% of oriented fibers made from mesophase pitch and 20% to 50% of a thermoplastic carbonizable binder. The resulting carbon article had a reduced longitudinal (with-grain) coefficient of thermal expansion.
U.S. Pat. No. 4,998,709 to Griffin et al. discloses a method of making graphite electrode nipples using carbon fibers derived from mesophase pitch added to blends of coke and pitch to produce an electrode pinstock. The invention adds from 8 to 20% of mesophase pitch based carbon fibers to 65% premium coke and 22 to 28% of a binder to form an extrusion blend and extruding to form a pinstock artifact. It is believed that the high degree of alignment of the carbon fibers is necessary to achieve the decrease in longitudinal CTE. However, there appears to be an undesirable increase in the transverse (against-grain) CTE, and lower strength. The undesirable effect of a higher transverse CTE results from the carbon fibers having a very low longitudinal CTE (xe2x88x921.5xc3x9710xe2x88x926xc2x0 C.) but a very high transverse CTE (about 5 to 8xc3x9710xe2x88x926/xc2x0 C.).
One major obstacle to attaining lower CTEs in both the transverse and longitudinal direction is the inability to homogeneously disperse the fibers in the electrode mix prior to extrusion to randomly orient the carbon fibers. Generally, when adding the fibers to the electrode mixes, it is preferable to have a fiber length no larger than that of the largest coke filler particles (about 1 inch). To achieve such a product, the carbon fibers are generally compacted with the use of a sizing material and then chopped into small bundles. Each bundle may contain up to about 20,000 mono-filaments. When these fiber bundles are added to electrode mixes containing coke and pitch or pre-blended with the coke prior to pitch addition, homogenous distribution of the bundles into individual mono-filaments is virtually impossible. The fibers cluster into balls or clumps when mixed with the electrode ingredients. FIG. 1 is a photomicrograph at 200xc3x97 magnification of a cross-section of a typical electrode mix of coke filler and pitch to which fiber bundles were added and mixed in a conventional manner. The fibers 10 are shown as small light colored elliptical shapes and are agglomerated in a background of pitch 15 along with coke particles 25. These agglomerated fibers lead to poor structure in the formed electrode and require high amounts of fibers to achieve desired property improvements. During extrusion, the needle-like coke filler particles align in a substantially longitudinal orientation. This provides a measure of reduction in the CTE in the longitudinal direction, but a likely increase in the transverse direction. Thus, it would be beneficial to find a method of dispersing the carbon fibers in a random orientation to lower the CTEs in both directions.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a method of utilizing carbon fibers to not only decrease the longitudinal CTE, but also the transverse CTE in carbon bodies, particularly graphite electrodes.
It is another object of the present invention to provide a method of dispersing carbon fibers in a pitch binder such that the fibers are randomly oriented to reduce both the transverse and longitudinal CTEs in a resultant graphite electrode.
A further object of the invention is to provide a method of utilizing the minimum amount of carbon fibers to achieve the aforementioned desired effects.
It is yet another object of the present invention to provide a carbon body having a reduced coefficient of thermal expansion.
It is still yet another object of the present invention to provide a graphite electrode having a reduced longitudinal CTE, and also a reduced transverse CTE.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The present invention is directed to, in a first aspect, a method of making a fiber pitch binder comprising the steps of: (a) providing a pitch having a viscosity of about 0.1 to about 5 poise; (b) providing carbon fibers in an amount of about 0.5 to about 10.0 wt. % of the pitch; and (c) admixing the fibers and the pitch to disperse the fibers into a fiber pitch binder. Preferably, step (a) comprises providing a pitch having a viscosity of about 0.1 to about 5 poise at a temperature of about 260xc2x0 to about 140xc2x0 C. Preferably, in step (c) the fibers are substantially dispersed into substantially single mono-filaments of a random orientation within the fiber pitch binder. Preferably, step (c) comprises admixing the fibers and the pitch by heating the fibers and the pitch to a temperature wherein the pitch has a viscosity of less than about 5 poise, followed by stirring at about 100 to about 1000 rpm for a sufficient time such that the fibers are substantially dispersed into substantially single mono-filaments which are randomly oriented within the fiber pitch binder. Upon substantial completion of step (c), the fiber pitch binder has a softening point of about 90xc2x0 C. to about 200xc2x0 C., a modified Conradson Carbon (MCC) value of about 50 to about 75%, and a viscosity of about 1 to about 50 poise at about 160xc2x0 C.
In yet another aspect, the present invention is directed to a method of making a fiber pitch binder comprising the steps of: (a) providing a pitch having a viscosity of about 0.1 to about 5 poise; (b) providing an amount of carbon fibers; and (c) admixing the fibers and the pitch to homogeneously disperse the fibers into a fiber pitch binder such that the fibers are dispersed into substantially single mono-filaments which are randomly oriented within the fiber binder pitch.
In still another aspect, the present invention is directed to a pitch based binder comprising an admixture of pitch having a viscosity of about 0.1 to about 5 poise at a temperature of about 260 to about 140xc2x0 C. with 0.5 to about 10.0 wt.% of carbon fibers based on a weight of the pitch, substantially homogeneously dispersed within the pitch as substantially single mono-filaments which are randomly oriented. The resultant admixture has a softening point of about 90 to about 200xc2x0 C., a MCC value of about 50 to about 75% and a viscosity of about 1 to about 50 poise at about 160xc2x0 C. Preferably, the admixture has substantially similar rheological behavior with respect to the rate of change in viscosity with temperature as the starting material pitch.
In a further aspect, the present invention is directed to a method of forming a carbon body comprising the steps of: (a) providing a binder comprising an admixture of pitch having a viscosity of about 0.1 to about 5 poise at a temperature of about 260 to about 140xc2x0 C. and about 0.5 to about 10.0 wt.% of carbon fibers based on a weight of the pitch, substantially homogeneously dispersed within the pitch as substantially single mono-filaments which are randomly oriented; (b) providing a filler; (c) mixing the binder having the carbon fibers substantially homogeneously dispersed as substantially single mono-filaments and the filler to produce a binder-filler mix; (d) shaping the binder-filler mix to form a shaped body; and (e) carbonizing the shaped body to form a carbon body. Preferably, step (a) comprises providing a binder having a softening point of about 90 to about 200xc2x0 C., a MCC value of about 50 to about 75%, and viscosity of about 1 to about 50 poise at about 160xc2x0 C.
In still yet another aspect, the present invention is directed to a carbon body having a substantially homogenous distribution of carbon fibers dispersed within the carbon body as substantially single mono-filaments which are randomly oriented, the carbon fibers present in an amount of about 1.5 wt.% to about 3.0 wt. % based on a weight of the carbon body.
In still yet another aspect, the present invention is directed to a method of making a graphite body having a reduced coefficient of thermal expansion comprising the steps of: (a) providing a binder comprising an admixture of pitch having a viscosity of about 0.1 to about 5 poise at a temperature of about 260 to about 140xc2x0 C. and about 0.5 to about 10.0 wt. % of carbon fibers based on a weight of the pitch, substantially homogeneously dispersed within the pitch as substantially single mono-filaments which are randomly oriented; (b) providing a filler; (c) mixing the binder and the filler to produce a binder-filler mix having a substantially homogenous dispersion of carbon fibers which are randomly oriented throughout; (d) shaping the binder-filler mix to form a carbon body; (e) carbonizing the carbon body; and (f) graphitizing the carbon body to form a graphite body. Preferably, step (f) comprises graphitizing the carbon body to form a graphite body having about 1.5 wt. % carbon fibers based on a weight of the graphite body, substantially dispersed throughout the graphite body as substantially single mono-filaments which are randomly oriented.
In still yet a further aspect, the present invention is directed to a graphite body having a substantially homogenous distribution of carbon fibers dispersed within the carbon body as substantially single mono-filaments which are randomly oriented, the carbon fibers present in an amount of about 1.5 wt. % to about 3.0 wt. % based on a weight of the graphite body.
In still yet a further aspect, the present invention is directed to a graphite body having a longitudinal coefficient of thermal expansion of about xe2x88x920.5xc3x9710xe2x88x926/xc2x0 C. to about 0.2xc3x9710xe2x88x926/xc2x0 C. as measured from about 25 to about 200xc2x0 C.
In still another aspect, the present invention is directed to a graphite body having a substantially homogenous distribution of carbon fibers dispersed within said graphite body as substantially single mono-filaments of a random orientation, said carbon fibers present in an amount of about 1.5 wt.% based on a weight of said graphite body.
In still another aspect, the present invention is directed to a graphite body produced by a method of: (a) providing a binder comprising an admixture of pitch having about 0.5 to about 10.0 wt. % of carbon fibers based on a weight of said pitch, substantially homogeneously dispersed within said pitch as substantially single mono-filaments of a random orientation; (b) providing a filler; (c) mixing said binder and said filler to produce a binder-filler mix having a substantially homogenous dispersion of carbon fibers which are randomly oriented throughout; (d) extruding said binder-filler mix to form a carbon body; (e) carbonizing said carbon body; (f) graphitizing said carbon body to produce said graphite body having about 1.5 to about 3.0 wt. % carbon fibers based on a weight of said graphite body, said fibers dispersed throughout said graphite body as substantially single mono-filaments of a random orientation.
In a final aspect, the present invention is directed to a graphite body produced by a method of: (a) mixing a binder comprising an admixture of pitch having a viscosity of about 0.1 to about 5 poise at a temperature of about 260 to about 140xc2x0 C. and about 0.5 to about 10.0 wt. % of carbon fibers based on a weight of said pitch, said fibers substantially homogeneously dispersed within said pitch as substantially single mono-filaments of a random orientation, with a coke filler to form a binder-filler mix; (b) extruding said binder-filler mix to form a carbon body; (c) carbonizing said carbon body; and (d) graphitizing said carbon body to produce said graphite body having about 1.5 to about 3.0 wt. % carbon fibers based on a weight of said graphite body, said fibers dispersed throughout said graphite body as substantially single mono-filaments of a random orientation.